1. Field of the Invention
The present invention is directed to an improved radiating cable having utility as transmitting and receiving antennas and as transmission lines.
2. Description of the Prior Art
Numerous types of radiating cables exist. Generally, they may be categorized as radiating coaxial cables or bifilar cables. The bifilar cables are unshielded and the radiating coaxial cables contain various types of apertures in the outer conductor to allow radiation. The apertured outer conductors are referred to as radiating sheaths and provide the means for coupling radio frequency energy between the path inside the sheath and the path outside the sheath. Some radiating coaxial cables additionally employ field perturbing elements which disturb the exciting field within the transmission line so as to enhance the radiating field, inductive elements for increasing the inductance of the outer conductor or drain wires which are laid over or under the outer conductor(s) and function as a ground connection. Such elements do not function as radiating sheaths since they do not serve to couple radio frequency energy between the paths inside and outside their position in the cable.
Many workers in the art have measured the performance of radiating coaxial cables and have found that they behave very similarly. Based upon these studies it has been determined that in order to obtain the desired radiation intensity, the apertures in the outer conductor must be so large that the attenuation of the propagation of the internal TEM signal increases dramatically along the transmission line and in some cases may even be fifteen times greater than that observed from a similar coaxial cable without apertures. However, even this high degree of attenuation is a significant improvement over bifilar radiating cables. As is well known, such attenuation severely limits the length that unamplified signals can be transmitted along the cable.
It is also known that the intensity of radiation from existing radiating cables, be they bifilar or coaxial, is dependent upon the environment of installation, i.e., underground, underwater, aboveground, etc. Here again, coaxial radiating cable out performs bifilar cable but remains environmentally sensitive.
A further problem which plagues coaxial radiating cable results from moisture ingression through the radiating apertures.